If you don't care about performance, we can make that code a lot shorter.

  float Q_rsqrt(float number) {
    return 1 / sqrt(number);
  }

The "fast inverse square root" is absolutely 100% all about performance. For it to make sense to use as a counter-example, you need to show alternate code that still meets the contract (the contract being: be as fast as this code), that is longer, and clearer.